By doing special mission available in some condition:
MOUNTAIN: move a boulder in north area in stream by the help of wood spirit
FIELD:help yoko to end her problem with her friend(yoko is near the entrance to the mountain)
4TH place(i forgot its name):give a curry maker in south of stream some fishes to make his curry
SWAMP:accessible from the 4th place after you finish your showdown with naoko. you might also have to found a culprit from a murder case
ANCIENT CAVE: found all pieces of fossil(river king lives here) -Answer by:patrick airleon